My biggest disagreement with the pet store guy is that you really do need to get a thermometer so you can shoot for the proper temperatures that have already been mentioned. I use a setup that is designed for monitoring weather, with a "main console" that has three channels for wireless remote digital probes that measure temperature and humidity, and I put one at the basking site, one at the cool end, and one in the middle of the tank. That way you can be sure if your temperatures are correct at all times. Others recommend something called an "infrared temp gun" (?), but I have no experience with those. Avoid the little stick on dial thermometers -- they're not accurate. But you do need something to give you accurate readings on the temperatures.
I also use a UV meter to check the output on my UV tubes about once a month, so I can replace them when the output starts to drop. Either the Solarmeter 6.2 or 6.5 can work well. The 6.2 reads UVB in microwatts/square centimeter, and the 6.5 gives a UV index which can be more useful for determining if your beardie is getting what he needs, but either one is adequate just for deciding when to replace the tubes. Unfortunately, both of them cost about $250 (last time I checked), so you can get by without a meter by replacing the tubes according to the manufacturer's recommendations, but I have found that there is a wide variation between different tubes -- some of them have a lower output when they're brand new than others, according to the meter.
I think the most important thing (as others have said) is to get your beardie to a qualified experienced reptile vet and follow the advice that you get from the vet to the best of your ability. And the vet's advice should supersede whatever you were told at the pet store.
I hope we've all been able to do more good than harm with helping you to get your beardie happy and healthy and keep him that way.
And BTW, thanks everyone for bringing it to my attention that it's not a good idea to leave a water dish in your beardie's enclosure at night. I've been using an upside down frisbee (too shallow to drown in and helps with humidity since it gets very dry inside Alaskan homes in the winter), but it never occurred to me that he could lay in it at night and get too cold. He has never done that so far, but better safe than sorry. I'm going to try to get in the habit of removing the "water dish" (along with the stale salad that I replace in the morning) from now on.